Cellular senescence and apoptosis are critical defense mechanisms that allow higher organisms, including humans, to escape oncogenic consequences. For example, expression of an oncogene (such as Ras) elevates the cellular levels of reactive oxygen species (ROS). A moderate increase in ROS leads to premature senescence, whereas a very high level leads to apoptosis. These two defense mechanisms are often disrupted during development of a tumor cell. The tumors of the hematopoietic lineage, however, often retain these functions; and they undergo apoptosis or senescence upon treatments with DNA damaging agents. In those tumors, disruption of the apoptosis or senescence programs leads to development of drug resistance. During the last funding period, we carried out detailed analyses on the mechanism by which the nucleotide excision repair protein DDB2, encoded by the XP-E gene, participates in DNA repair. Surprisingly, during those studies, we observed evidence that DDB2 plays important roles in premature senescence and DNA damage-induced apoptosis. Cells lacking DDB2 fail to undergo senescence induced by culture shock or stress and they are resistant to apoptosis induced by several genotoxic agents. In the current proposal, we plan to investigate the roles of DDB2 in the pathways of ROS-inducing oncogenes, such as Ras and Myc. We will test the hypotheses that DDB2 plays important roles in tumor development and that it is critical in sensitizing hematopoietic tumors to the DNA damaging drugs, which are used in chemotherapy. The specific aims are: 1. How does ROS activate expression of DDB2? Does oncogene-induced accumulation of ROS require DDB2? Does DDB2 accelerate or delay oncogene-induced tumor development? 2. Does DDB2-deficiency confer resistance to chemotherapeutic drugs? Does re-expression of DDB2 sensitize resistant cells to chemotherapy? 3. How does DDB2 overcome the apoptosis-inhibitory activity of Mdm2?